In drilling a well or other similar borehole, a drill bit is operatively coupled, usually by a drill string, to a drive which rotates the drill bit to cause the drill bit to bore into the earth. A drilling fluid, or drilling mud, is circulated through the borehole annulus. The drilling fluid passes through the drilling string and to the surface through the drill bit for cooling and lubricating the drill bit and for carrying rock cuttings generated by the cutting action of the bit to the surface. The drilling fluid may be a gas, but more typically is a liquid.
Many liquids suitable for use as drilling fluids are known. Some drilling fluids are oil-based. Such oil-based fluids suffer from a number of drawbacks, particularly, their adverse effects on the environment. Oil-based fluids also can be costly to purchase and to dispose of.
Other drilling fluids are water-based or constitute an emulsion of oil in water or water in oil. Such fluids often are inexpensive compared to oil-based fluids, and are less costly to dispose of. Water-based fluids also pose less of a risk to the environment than do oil-based fluids. One drawback associated with water-based fluids is that the water in the fluid tends to promote borehole instability, particularly when shale is encountered in the drilling process. The water-based fluid may adsorb and absorb into pores in the shale, thus causing the shale to swell and thereby tending to cause the borehole to collapse.
In recognition of this drawback, water-based drilling fluids have incorporated an alkyl glycoside, such as methyl glucoside, into the drilling fluid. It is known that methyl glucoside serves as a borehole stabilizing agent that functions by inhibiting shale swelling. The mechanism of action of methyl glucoside is not fully understood, but is believed to be associated with gel formation or an ion exchange phenomenon. It is generally believed that the methyl glucoside, in adding gel strength to the drilling fluid, causes more rapid formation of filter cake on the borehole wall thereby allowing less water filtrate to reach the surrounding shales. This is believed to reduce swelling and sloughing of the shale.
While known drilling fluids that include methyl glucoside are satisfactory, there remains room for improvement in such fluids in the area of borehole stability. It is a general object of the invention to provide a drilling fluid that is suitable for use in borehole drilling. Another general object is to provide a drilling apparatus and process that incorporate the drilling fluid of the invention.